One of the broad objectives in the study of hemoproteins, and metalloproteins in general, is to understand the means by which a protein environment modulates the chemical reactivity of a metal-containing prosthetic group. A complementary objective is to understand the involvement of the heme prosthetic group in hemoglobin function-regulating conformational changes, and to understand the complex process of hemoglobin ligation. Pursuit of these objectives involves the application of a variety of measurement techniques to the native proteins and their model compounds. Ligand-binding, redox properties, electron paramagnetic resonance, and resonance Raman spectra of mammalian and fish hemoglobins can be employed to clarify the process and mechanism of hemoglobin cooperative ligand binding. Electron-nuclear double resonance studies of the hemoglobins, of transferrin, of other proteins, and of model compounds can be used to compare the properties of protein-bound metal-ion, and to clarify the effects of the apoprotein on the structures of a prosthetic group. The same is true for studies of heme-ligand photosensitivity. In addition, we significantly expand these studies through the use of metal-substituted proteins, and have been examining metal-substituted hemoglobin, myoglobin, peroxidase, and cytochrome P450. By employing the full range of metalloporphyrin proterties, using cobalt, manganese, zinc, and other metals as well, we are able to prepare proteins whose physical and chemical characteristics are altered in significant and interpretable ways, expanding our ability to directly observe the influence of a protein environment on the physicochemical properties of a prosthetic group.